CS211362B2 - Non-alloyed of low alloyed bainitic hardenable steel alloy and method of making the same - Google Patents

Abstract

The invention provides a steel (or a steel article such as a fastener for use in an explosive-powered fastener- driving tool) which is both tough and strong, economically, and comprises: 0.50 to 1.00% carbon 0.10 to 1.50% silicon 0.25 to 1.50% manganese up to 0.045% phosphorus up to 0.045% sulphur 0.20 to 0.90% molybdenum and/or 0.20 to 2.00% chromium, the remainder being iron, possible slight-alloying elements and impurities, having been subjected to a bainite hardening treatment in the region of or at temperatures above the martensite point in the range 0 to 100 DEG C above said point, until a maximum transformation of 85% has been achieved. The invention provides, also, a corresponding method.

Description

BACKGROUND OF THE INVENTION The present invention relates to a low alloy or low alloyed balnitically curable steel alloy with a slow. a low point conversion to martensite that is tough and high strength.

The invention further relates to a production method. . of this steel.

Steel and iron alloys are known which are produced using a plurality of valuable alloying additives and which at the same time have a high strength of 1 toughness. However, these steels are only usable for special purposes because of their high production costs, also due to technical reasons. It is understandable that many developmental tasks have been to find a steel or iron alloy that has high strength and toughness, but which can be produced in a more cost-effective manner, at a lower production cost, but has not yet achieved satisfactory results.

For example, high alloy steel of high strength and toughness is known which contains at least 0.03% carbon, at least 0.10% by weight in a concentration by weight. silicon, at least 0.10% manganese, 0.010% phosphorus, 0.010% sulfur, 11.5 to 13.5% cobalt, 3.00 to

4.50% molybdenum, 17.0 to 18.5% nickel, 0.20% aluminum, 1.50 to 2.00 ·% titanium. (Steel of this composition is designated in the International Steel Identification Key 1.6356 and bears the symbol X2 NiCoMoTi 18 12 · 4). Another type of such steel is a steel containing, by weight, at least 0.03% carbon, at least 0.10% silicon, at least 0.10% manganese, 0.010% phosphorus, 0. 010% sulfur, 7.00 to 8%, 50% cobalt, 4.60 · 5.20 · molybdenum, 17.0 to 19.0 percent nickel, 0.0.5 to 0.15 ·% aluminum, 0.30 to 0.60% titanium, 400-3% boron, 0.05% calcium and 0.02. ·% Zirconium. (This steel is referred to in international key material number 1.6359 and bears the symbol X 2 NiCoMo 18 8 5). However, these steels are difficult to form, difficult to machine, and low in economic due to the high cost of alloying ingredients.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide such steel and products made thereof which are economically feasible and which, independently of the presence of significant proportions of valuable alloying additives, simultaneously have high toughness and high strength, or which have extremely high strength values.

Another object is to provide a suitable method for producing this steel by which the steel with high strength and toughness can be produced or by which it is possible to produce articles of this steel. These articles are, on the one hand, mass consumption products, such as bolts, stop bolts and nails of conventional shapes for direct fastening of parts, which can also be driven into high-strength structural steel elements.

This object is essentially achieved according to the invention, the object of which is an unalloyed or low alloyed low-conversion, low-point bainitically curable steel alloy transforming into martensite, the object of the invention is that the steel alloy contains from 0.50 to 0.5% by weight. 1.00 ·% · carbon, 0.10 to 1.50% · silicon, 0.25 to 1.50% manganese, 0 to 0.045% phosphorus, 0 to 0.045% · sulfur, 0.20 to 0.90 · % of molybdenum · and / or 0.20 to 2.00 ·% of chromium, with the remainder being iron and common impurities, which steel · alloy was subjected to bainite curing in the martensite conversion zone and / or in the temperature zone · directly · above this zone and in which the bainitic curing has a degree of conversion not more than 85%.

For example, one part of chromium can be used instead of one part of molybdenum. Generally, molybdenum has priority over chromium. In special cases, the lower limit of the manganese content may be 0.25%.

The steels according to the invention produced by the process according to the invention and having the above-mentioned composition are steel materials with sufficiently slow conversion, which are prerequisites for isothermal bainitic conversion and which have, as far as possible, a low conversion temperature to martensite. In an autocatalytic network from Austin and Rickett, these steels generally have a break in the otherwise straightforward conversion kinetics. There is a deposition that clearly affects the toughness of the steel. With the present invention, a steel having a high strength and toughness is obtained with a course in which the unfavorable sections which adversely affect the toughness of the steel are removed. In addition, an increase in impact resistance, a reduction in the heating time required for conversion and thus energy savings are achieved.

The invention is based on the discovery that steels and other iron alloys, and articles made therefrom having initially unfavorable properties, acquire higher strength and greater toughness when subjected only to imperfect bainitic curing, i.e., a heat treatment that lasts until conversion , of the order of 80%, at Veplotaches in or above the martensite conversion temperature range just above the mairtensite conversion point.

The transformation point into martensite is usually in the range from 180 ° to 280 ° C, in particular from 180 ° to 23 ° C ^! ° C and in particular in the range of 185 to 210 · ° C. For the purposes of the invention, the temperature in the martensite conversion zone is to be understood to mean temperatures in the range between 10 ° C above and 10 dc below the point at which the martensite is formed. The thermal region above the martensite conversion point at which imperfect bainitic curing is carried out is generally in the range of 0 ° to about 100 ° C, in particular up to · 50 ° C, for example in the range of 5 to 50 ° C above the martensite formation point. In general, it turns out that • to the extent. of the invention belonging to the temperature region above the martensite transformation point, in which the imperfect mine of the nitic curing of the steel according to the invention can be carried out, the narrower the higher the martensite formation point and the wider the martensite transition point . Preferably, the temperature for imperfect bainitic curing is less than 270-280 ° C, especially less than 260 ° C.

Particularly favorable results are obtained when using steels with a martensite conversion point below the values given in the previous section. If the starting steels do not have a suitable structure to perform imperfect bainitic curing, it is of course necessary to first carry out a pretreatment, for example heat, to form an austenitic structure of the steel. The possibly small proportions of alloying elements present in the steel may be, for example, vanadium, tungsten, niobium, boron and the like, which may occur in a concentration by weight of up to 0.2%. This may affect some steel properties. However, the presence or absence of these alloying elements has no greater effect on the solution of the invention.

The conversion can take place up to an area of not more than about 85% based on the maximum, i.e., exhaustive, conversion. The degree of conversion is predominantly between 55 and 85%, with a range of between 75 and 85% being preferred, and in particular 80% ·%. The degree of conversion can be readily determined by measurement methods with sufficient accuracy using circumstances, that the bainitic conversion is associated with a positive change in length. Therefore, the kinetics of the conversion and thus the degree of conversion at the temperature just reached can be determined in a simple manner by a dlatometric measurement on a steel sample. Thus, eighty percent conversion is understood to mean that eighty percent of the maximum elongation that has reached full bainitic conversion at the same temperature.

According to a preferred embodiment of the invention, the steel alloy having a relatively high strength and toughness comprises at a weight concentration of 0.57 to 0.75% carbon, 0.15 to 0.35% silicon, 0.70 to 0.85% manganese, up to 0.035% phosphorus, 0 to 0.35% sulfur, 0.20 to 0.50% molybdenum and / or 0.20 to 1.00% chromium, the remainder being iron, possibly minor proportions of alloying elements and common impurities.

Also applicable to this steel are the facts mentioned in the previous section in relation to the conditions and characteristics of the conversion. This steel also has advantageous desirable properties when the conversion is carried out in a martensite-forming temperature range or in the region of 0 to 50 ° C above the martensite conversion point and when the conversion is 75 to 85% of the maximum value measured by dilatometric kinetics transformations.

The steel or iron alloys according to the invention and thus articles made of this steel have a toughness of more than 70 J, up to 85 J or even 90 J, measured on a non-notched circular test specimen, and a hardness of at least 57 HRC, especially 58 HRC and higher , reaching 60 and 61 HRC.

The invention also provides a process for producing this improved steel and articles made therefrom, i.e., slow conversion bainitically curable steels, which are low alloyed or unalloyed and have a low martensite conversion temperature, the principle of which is that the bainite hardening is carried out at a temperature in the region of the martensite conversion point or in a region directly adjacent thereto upwards, in particular in an area which is 0 to 100 ° C, in particular up to 50 ° C, for example 5 to 50 ° C higher than the point the conversion to martensite, wherein the curing is interrupted when 85% of the deformation is reached, for example 55 to 85%, in particular about 75 to 80%, based on the dilatometric measured maximum conversion. Preferably, steels and iron alloys having said composition are used to carry out the process. Here too, the incidental presence of minor proportions of alloying elements is not decisive for achieving the desired results and properties of the steel.

The articles and products of advantageous properties can be prepared from the steel according to the invention by conventional manufacturing processes, in particular by cold forming. However, it is equally possible to produce these articles from untreated steel but having said material composition, and then to carry out the processing according to the invention on finished articles in which the original untreated steel is converted to high-strength steel of high strength and toughness. In this case, the particular advantage of the process according to the invention is that, in contrast to the manufacturing conditions for high-alloy steels, which are quite demanding, large-scale production of large series of articles of common consumption can be carried out in a more economical manufacturing process. These bulk items include, in particular, fasteners, such as screws, studs, bolts, direct fasteners and nails, rivets, dowels, dowels and parts thereof, tools such as drills, drill bits, screwdrivers, saws, punches, punches, mandrels, machine parts such as: · safety pins, · springs, disc springs, valves, valve guides, piston rings, shafts, axles, cotter pins, couplings, washers, slats, etc., components of military equipment and weapons, armor plates , fittings for building, marine, furniture and for sports equipment, such as chains, suspension hooks, · ski edges, components of optical · measuring instruments, various semi-finished products such as steel strips, wires, sheets, rods, tubes and the like.

Particularly suitable for the purposes of the invention are steels of the group of spring steels comprising, for example, in a weight concentration:

Steel designation: C% Si% Mn o / o Cr% In O / o 62 SiMnCr 4 0.62 1.0 1.0 0.6 - 58 CrV 4 0.58 0.3 1.0 1.1 0.1 50 CrV 4 0.50 0.3 1.0 1.1 0.1 62 SiCr 5 0.62 / 0.72 1.3 0.5 0.5 - or steel from the processing steel group cold, for example: Designation C% Si% Mn ° / o Cr% V o / o steel: 100 Cr 6 1.0 0.3 0.3 1.5 -

optionally non-alloy steels from the group of tool steels or carbon-alloyed steels, for example:

Steel designation: C o / ₀ Si% M n o / o P and S% C / f, k, g, / 60 0.57 to 0.65 0.25 0.25 0.045 max C / f, k, g, / 67 0.67 0.25 0.70 0.045 max C / f, k, g, 770 0.70 0.25 0.60 0.045 max C / f, k, g, / 75 0.75 0.25 to 0.50 0.60 to 0.80 0.045 max C / f, k, g, / 85 0.85 0.90 0.60 to 0.80 0.045 max C / f, k, g, / 100 1.0 0.25 0.60 to 0.80 0.045 max

Examples of steels according to the invention are illustrated in the following examples.

Example 1

The starting material is steel containing, by weight, 0.73% C, 0.85% Mn, 0.20% Si and 0.29% Mo, cold drawn into a wire shape, soft annealed, slightly tightened, with a strength of approx. 800 MPa; this steel is treated for 20 minutes at 240 ° C and 40 minutes in air after heat treatment at 860 ° C and then divided into sections of length 1 = 50 mm and diameter φ = 4,5 mm.

The temperature of the martensite conversion point is 220 ° C, measured by a high-speed dilatometer, and the conversion time in minutes at a conversion temperature of 240 ° C and degrees of conversion of 85%, 95%, 99% and 100 ° / o - relative to dilatometrically measured length changes :

% ..... 55 minutes% i ..... 75 minutes

% ..... 110 minutes

100% ..... 210 minutes

The products had a hardness of 59 HRC and impact resistance:

at 100% length change -

X - 51.5 J (n. = '20 tests) at 86% length change = '

X - 80 J (n = 20 tests)

From the above results it is evident that increasing the impact resistance compared to the values (Δ 1 = ^1,100%) attainable in the known isothermal bainite contention is not the only advantage of the invention, but that also achieves a considerable shortening the heating times required к conversion and thus saving energy.

Example 2

After subjecting the finished fastener, such as a nail or bolt, to the same heat treatment as in the previous example, the finished product has a strength of 58 HRC and at the same time good toughness, and can be bent up to 90 ° without breaking.

Example 3

In the manufacture of high tensile and high tensile springs, 80 mm diameter helical springs are first formed from the starting material consisting of a wire having a diameter d i = 4.5 mm and then subjected to the same heat treatment as in the previous examples.

Claims (4)

OBJECT OF THE INVENTION A low alloy or low alloy bainitically curable steel alloy having a slow conversion and a low conversion point to martensite, characterized in that it contains in a concentration by weight of 0.50 to 1.00% carbon, 0.10 to 1.50% silicon, 0.25 to 1.50% manganese, 0 to 0.045% phosphorus, 0 to 0.045% sulfur, 0.20 to 0.90% molybdenum and / or 0.20 to 2.00% chromium, the remainder being iron and the usual impurities, which steel alloy was subjected to bainite curing in a temperature zone of the martenslt transformation, or in a temperature range directly above this zone, in which the bainite curing reached a conversion rate of not more than 85%. 2. An unalloyed or low-alloy bainitically curable steel alloy according to claim 1, characterized in that its heat treatment has been carried out in a temperature zone of martensite or in a temperature zone of 0 to 100 ° C, in particular 0 to 50 ° C. C, in particular 5 to 50 ° C above the martensite conversion point. 3. An unalloyed or low-alloy bainitically curable steel alloy according to items 1 and 2, characterized in that its degree of conversion reaches 55 to 85%, in particular 75 to 85%, in particular around. 80%. 4. A process according to claim 1, wherein the steel alloy is subjected to a bainite hardening in the temperature zone of the martensite conversion or in the region of -0 to 100 DEG. C, in particular 0 to 50 ° C, in particular 5 to 50- ° C above the temperature of the transition point into martensite, and · interrupts at the latest 85%, in particular 55-85%, especially 75-80% conversion based on dilatometrically measured maximum conversion.